This invention relates to self-regulating semi-conductive flexible heating elements, and in particular to flexible, homogeneous carbon polymeric heating elements configured to resist water and chemical damage.
Flexible homogeneous carbon polymeric heating elements have been employed in a number of applications, particularly in heating floors, melting snow, and deicing. These elements typically include an elongate web of an electrically conductive plastic, such as a polyethylene and carbon black mixture. There are bus conductors embedded in the web, extending longitudinally adjacent each edge of the web. These bus conductors may be, for example, a braided wire. The bus conductors allow a potential to be applied transversely across the web, thereby generating heat. The elongate web is extruded as a flat heating element. To increase the flexibility of the web and decrease the cross sectional area of the web, a plurality of slots can be cut transversely across the web.
At least some known heating elements are made from an electrically conductive homogeneous low density polyethylene. These heating elements are capable of operating at low voltages (e.g., 30 volts or less), and are self-regulating because as the temperature of the element increases, the resistance increases, decreasing the current and thus the heat being generated. Moreover, as compared to alternative heating systems, the use of these heating elements in floors may provide a more even heat distribution, greater comfort, less temperature stratification, better control, increased ability to provide zoning, and/or the elimination of forced air which can circulate dust and germs. These heating elements are also capable of operating at line voltage (e.g., up to 277 volts) for concrete applications.
However, exposure to water, chemicals, and other environmental conditions may damage at least some known heating elements, reducing the durability, conductivity, and/or efficiency of the damaged heating elements. Further, at least some known protective liners may delaminate over time, allowing water and/or chemicals to reach and damage the heating element. Specifically, the water and/or chemicals may encapsulate the carbon molecules in the heating element, impairing the ability of the carbon to transfer electricity. Moreover, at least some protective liners themselves may interact adversely with the heating element, choking the carbon and inhibiting transfer of electricity.